A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some therapies include the delivery of electrical signals, e.g., stimulation, to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient. Some medical devices may be “leadless” and include one or more electrodes on an outer housing of the medical device to deliver therapeutic electrical signals to organs or tissues and/or sense intrinsic electrical signals or physiological parameters of a patient.
Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of therapeutic electrical signals or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient, while in other cases percutaneous leads may be implanted and connected to a medical device housing outside of the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices. Leadless medical devices are typically implantable medical devices positioned within or adjacent to organs or tissues within a patient for delivery of therapeutic electrical signals or sensing. In some example, leadless implantable medical devices may be anchored to a wall of an organ or to tissue via a fixation mechanism.
Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical signals to the heart, e.g., via electrodes carried by one or more medical leads or via electrodes on an outer housing of a leadless implantable medical device. The therapeutic electrical signals may include pulses for pacing, or shocks for cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of therapeutic signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate therapeutic electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing stimulation to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation.
In general, implantable medical devices require a small housing form factor to enable an unobtrusive implantation within a patient. In the case of leadless implantable medical devices, the housing form factor must be extremely small to enable implantation within or adjacent to organs or tissue. For example, a leadless pacemaker may be implanted directly into a ventricle of the heart. Battery usage is always a concern when designing implantable medical devices, but this concern is increased for small form factor devices that can only accommodate a small battery canister.
Currently, many implantable devices attempt to minimize battery drain by means of capture management testing, as described in U.S. Pat. Nos. 5,601,615, 5,766,230, 6,553,259, 7,280,868, 7,457,666, and 7,761,162, incorporated herein by reference in their entireties. Such tests determine the pacing pulse threshold parameters (typically voltage and pulse width) necessary to capture the chamber of the heart being paced. These tests are also referred to as threshold tests.
The devices typically thereafter set the actual parameters to a higher energy level than the determined threshold parameters, typically to a higher voltage. By this mechanism, the devices provide a safety margin which decreases the likelihood that changes in the underlying condition of the patient's heart will result in a loss of capture. Such capture management tests may be performed according to defined pre-programmed schedules or in response to events indicating that capture is no longer reliably occurring.
Correspondingly, many devices include the associated capability to detect loss of capture. Such devices are disclosed in the patents cited above. Actual loss of capture may be detected on a beat to beat basis or by changes in detected cardiac rhythm. Detected loss of capture may trigger the performance of a threshold test, as discussed in the above-cited patents. The result will typically be a resetting of pacing parameters to parameters that provide the defined safety margin or by resetting to the maximum energy level deliverable by the device, whichever is less.
Many current devices employ capture management operations such as threshold measurement tests and safety margin checks as described in the above cited patents. In many cases, the presence of a stable intrinsic cardiac rhythm is a prerequisite to successful testing. Emergence of the patient's underlying rhythm may in some cases cause such capture management tests to fail. As a prerequisite to performing a threshold test or safety margin check, it is therefore desirable to first determine that the patient's underlying heart rhythm is stable. One mechanism for assessing stability for this purpose is set forth in the above-cited '868 patent. The present invention is directed to an improvement to such a stability check.